Control system



May 27, 1941.-

G. FISHER CONTROL SYSTEM Filed Aug. 21, 1939 Patented May 27, 1941CONTROL SYSTEM George H. Fisher, St. Louis Park, Minn., assignor toMinneapolis-Honeywell Regulator Company, Minneapolis, Minn., a.corporation of Delaware Application August 21, 1939', Serial No. 291,171

3 Claims.

The present invention is directed broadly to a control system forcontrolling the value of a condition, and more particularlyto a systemin which a condition responsive element energizes and deenergizes atimer which in turn actuatesthe control device for the condition to becontrolled.

It is one of the objects of the invention to maintain a condition atsubstantially the desired value by arranging the control system so thatwhen the condition decreases to a value slightly lower than the desiredvalue the condition responsive element will render the control deviceoperative to increase the value of the condition, and a predeterminedtime after the condition value has risen above the predetermined lowvalue the control device will be rendered inoperative whereby thecondition will increase to the desired value and will not rise way abovethat value. This is accomplished by having the condition responsiveelement energize a timer to render the control device operative at thepredetermined low value of the condition and de-' energize the timer torender the control device inoperative when the element responds to apredetermined higher value. When the condition value rises above thepredetermined low value an auxiliary control device is energized forcausing the condition responsive element to falsely assume the positioncorresponding to the predetermined higher condition value after apredetermined time, thereby rendering the control device inoperative.

A further object is to apply this control system to a fuel burner systemin which the timer controls an electric fuel feeding device and alsounlatches a safety switch to lock the system out after a predeterminedtime interval unless combustion is established first. Such a system isdisclosed in the copending application of John M. Wilson Serial No.253,338 filed January 28, 1939. In the present system the timer alsoestablishes the circuit to the auxiliary control device therebyrendering it capable of energization by the condltion responsive device,2

The invention will be illustrated and .described in connection with atemperature control system in which the condition responsive element isa thermostat, but it will be understood that the present invention maybe applied to other control systems such as pressure and humidity, forexample. The. auxiliary control device need not necessarily aifect thecondition responsive element by increasing the condition value to whichit responds. It may also move the element or its fixed at one end andwhich carries at its free end a pair of contact blades l3 and I4. Thecontact blades are so arranged that on a decrease in temperature theblade l3 will first be moved into engagement with the stationary contactl5 and as the temperature continues to decrease the contact blade M willbe brought into engagement with the stationary contact l6. Blade l3flexes to permit the blade H to engage the contact l6 after the blade l3has engaged the contact I5.

Indicated generally at 20 is a safety switch comprising a resilientcontact blade 2i which is connected at one end to the bracket 22 andcarries atits free end a contact 23. The blade 2| is biased downwardly.Resilient contact blade 24 is connected at one end to the bracket 25,carries at its free end a contact 26, and is also biased downwardly.Contacts 23 and 26 are normally held closed by means of the insulatingbutton 21 carried on the free end of the bimetallic timer 28 whose otherend is fixed to the bracket 29. In the cold position of the bimetallictimer 28 the button 21 holds contact 26 in engagement with contact 23but as the timer is heated by means'of the electric-a1 heater 30 itwarps toward the left and blade 24 will drop off the end of the button21 after a predetermined period of time and permit the contact 26 toseparate from the contact 23. The bimetallic timer 28 upon cooling willbe unable to reclose the contacts 26 and 23. They must be manuallyreclosed by means of the manual reset member indicatedat 32. The resetmember 32 comprises a long leg 33 and a short leg 34' which aresimultaneously moved by'means of the stem 35 which is slidably supportedin the stationary support 36. The stem 35 is manually raised by means ofthe knob 31 which first raises the contact blade 21 thereby holding thecontact 23 out of the path of movement of the contact 26 and then theshort leg 34 raises the contact blade 24 and relatches it on thebutton'2l'providing the bimetallic timer 28 has returned to coldposition. The knob 31 is then released at which time contact 23 ispermitted to reengage the contact 26.

The movable end of the bimetallic timer 28 also carries a contact 48which in the cold position of the timer 28 engages the stationarycontact 4|. Opposite the contact 48 the timer 28 carries the movablecontact 42 which is adapted to cooperate with contact 43 mounted uponthe re silient blade 44 as the timer warps toward the left as viewed inthe drawing. Also carried by the bimetallic timer 28 and insulated withrespect thereto is a movable contact 46 which cooperates .with contact41 as .the timer warps toward the left. Contact 41 is carried by theresilient blade 48.

Pivoted at 58 is a lever 5| which carries at its free end a mercuryswitch 52. A link 53 is pivotally connected at one end to the lever 5|and at the other end to the bimetallic timer 28 in such a manner thatwhen the bimetallic timer is in its cold position the link 53 cooperateswith the lever,

5| to hold the mercury switch 52 in open position and as the timer heatsup and warps toward the left the link 53 rotates the lever 5| in aclockwise direction to close the circuit through the mercury switch 52.Y

Indicated generally at 55 is a combustion responsive device comprising amovable switch arm 56 and a stationary contact 51. The switch arm 56 isadapted to be rotated by means of a combustion responsive device such asa stack thermostat for example. The switch arm 56 is connected with thethermostat by means of a slip friction device so that the: switch isactuated by the burner motor.

The power for energizing the above control system is supplied from thelines 68 and 6| to which the primary 62 of the transformer 63- isconnected at on end by means of the conductor '64. The other end of theprimary 62 is connected by means of conductor 65, the normally closedsafety switch 28 and conductor 66 to the other supply line 6|. Thesecondary winding 61 of the transformer 63 supplies low voltage currentfor the bimetal timer and room thermostat.

Assuming that the temperature in the zone to be controlled is at orabove the desired value the elements of the control system will be inthe positions shown in the drawing. It will be noted that at this timethe combustion responsive switch 56, 51 is open, the bimetallic timer 28is in its cold position and therefore holding the burner motor switch 52open, and the room thermostat II is holding both of its switches in openposition. On a decrease in room temperature the switch blade |3 willfirst engage the stationary contact l5. Closure of this switch will notcomplete an electrical circuit at this time due to the fact that theswitch l4, I6 is open and the switch 48, 41 is open. On a continueddecrease in room temperature the switch blade I 4 willbe brought intoengagement with the stationary contact l6 at which time the followingcircuit will be closed; from the secondary winding 61 of the transformer83 through conductor 18 to-contact |5, switch blades l3 and I4, contactI6, conductors 1|, 12 and 13, electric heater 38, conductors 14 and 15,protective resistance 16 and conductor 11 back to the other side of thesecondary winding 61. This circuit energizes the electric heater 38causing the bimetallic timer 28 to warp toward the left closing theswitch contacts 46 and 41 and also closing the burner motor switch 52.

Closure of the switch contacts 46 and 41 set up a holding circuit aroundthe switch |4, |6 which may be traced as follows: secondary winding 61,conductor 18, contact |5, switch blade l3, bimetallic element l2,conductor 18, auxiliary .heater 19, conductor 88, switch contacts 46 and41, resilient blade 48, conductors 8|, 12 and 13, electric heater 38,conductors 14 and 15,- protective resistance 16 and conductor 11 back tothe other side of the secondary winding 61. It 7 will be noted that thelatter circuit includes an auxiliary electric heater 19 located adjacentthe bimetallic element l2. This heater will not be energized at thistime because the original energizing circuit for the heater 38 is stillclosed at l4, l6, which circuit shunts out the heater 19.

Closure of the mercury switch 52 energizes the burner 58 and theignition device 59 by means of the conductor 66 which is connectedto'supply line 6|, the normally closed safety switch 28, conductor 83,mercury switch 52, conductors-84 and 85, burner motor 58 and conductors86 and 81 back to the supply line 68. The ignition device 59 isconnected in parallel with the burner motor 58 by means of theconductors 88 and 89.

As the burner motor and the ignition device are energized combustionwill normally be established which will cause the combustion responsivedevice 55 to move the switch arm 56 into engagement with the stationarycontact 51. As the bimetallic timer continues to warp to the left underthe influence of the electric heater 38 it will eventually cause themovable contact 42 to engage th stationary contact 43. This willestablish a shunt circuit around the electric heater 38 from theconductor 12 through conductor 98, switch arm 56, contact 51, conductor9|, switch blade 44, contacts 43 and 42, bimetallic timer 28 and bracket29 back to the conductor 15, This shunt circuit will deenergize theheater 38 causing bimetallic timer to cool off, warp to the right, andmove the contact 42 out of engagement with the contact 43 therebybreaking the above shunt cir-' cuit. This will energiz the heater 38causing the timer to warp to the left again and reclose the shuntcircuit. This action will continue, and during all this time the burnermotor switch 52 will be held in closed position to maintain combustionwithin the furnace.

As a result of the combustion produced within the furnace thetemperature at the thermostat II will begin to increase and thebimetallic element |2 will cause the switch arm H to disengage thestationary contact 6. This breaks the original energizing circuit forthe electric heater 38 and the circuit for this heater must now passthrough the bimetal element 2, the auxiliary heater 19, conductor 88,contacts 46 and 41, resilient blade 48, conductors 8|, 12 and 13 andelectric heater .38. -During this time the bimetallic timer 28 is bladel3 will be moved out of engagement with the stationary contact l5. Itwill be noted that when contacts 42 and 43 are open the heater 19 is inseries with resistances 30 and 15 and when contacts 42 and 43 areclosedheater 19 is in series with resistance 15 only. The current valuethrough heater 19 is therefore not steady, but the heat emitted by theheater has a tendency to average out and produce a practically even andregular effect on the thermostat I2. When this occurs the electricheater 30 will be deenerg'ized and the timer 28 will cool and openswitch 46, 41. The heater 30 cannot now be reenergized until thethermostat l2 recloses the original energizing circuit at the switch I4, [5 because the holding circuit has been broken at 46, 41. Cooling ofthe bimetallic .timer 28 also opens mercury switch 52 which shuts downthe burner motor and the ignition device.

It will be seen. that the original energizing circuit for the heater 30for the timer 28 passes through the switches l3, l5 and I 4, l6 inseries and that when the timer heats up it closes the switch 46, 41establishing a holding circuit for the heater 30 independently of theswitch l4, it so that this heater cannot be deenergiz'ed until thethermostat l4 separates contact blade l3 from the stationary contact 15.Further it will be noted that as the thermostat l2 initially responds toan increase in temperature resulting from the combustion within thefurnace it opens the switch l4, 16 which causes the energization of theauxiliary heater 19. This heater raises the tempera- .ture of thebimetallic element l2 above that of the surrounding atmosphere to thepoint where it causes the arm l3 to separate from the stationary contactl5 causing the bimetallic timer 28 to cool off and shut down the burnermotor and ignition device. The size of the heater 19 predetermines thelength of time required to cause the thermostat I2 to open the switchI3, I 5 following the opening of switch l4, IS.

The above description covers the normal operation of the control system.In the event combustion is not established after the bimetallic timer 28has warped sufiiciently to close the switch 52, then the combustionresponsive device 55 will not close the switch 56, 51- and as a resultthe shunt circuit around the heater 30 will not beestablished when thetimer 28 closes the contacts 42, 43. The bimetallic timer 28 willtherefore continue to warp toward the left pressing back the blades 44and 48 and after a predetermined time interval the flexible blade 24will drop off of the button 21 disengaging the contacts 26 and 23thereby breaking the circuit to the primary 52 of the transformer 63 andto'the burner motor and the ignition device. The parts will remain inthis position until they are manually reset as described above. It maybe pointed out that the manual reset is of the trip free type. In otherwords, the switch contacts26 and 23 cannot be manually held closed bypushing in the button 21 and can be closed only if the bimetallicelement 28 is in its cold position.

closed position this system will be unable to start up because of ashunt circuit around heater 30.

This circuit comprises conductor 90, switch arm 56, contact 51,conductors-9| and 55, contacts 4| and 40, bimetallic element 28 andbracket 29 to conductor 15. This prevents the system from starting whenit has no protection for ignition or combustion failure.

V As certain changes and modifications of this invention may occurl tothose who are skilled in the art it is to be understood that I intend tobe limited only by the scope of the-appended claims and not by thespecific embodiment disclosed.

I claim as my invention:

1. In a fuel control device, in combination, an electrically operablefuel supply controlling device, a circuit therefor, an electric timingdevice, connections by which said timing device, when electricallyoperable fuel supply controlling device, a circuit therefor, an electrictiming device, connections by which said timing device, when energized,first closes said circuit and thereafter opens it again, meansresponsive to combustion conditions cooperating with said timing deviceto prevent the reopening of said circuit if combustion is establishedfirst, a condition responsive element, means controlled by said elementas said condition decreases to a first value for energizing said timingdevice, means controlled by said element as said condition increases toa second higher value for deenergizing said timer, and an activatorrendered operative by said condition responsive element as saidcondition rises above said first value for causing said element to as-If combustion should fail for any reason whilethe system is running thecombustion responsive device will open the switch 55, 51 so that theheater 3!] will remain continuously energized until the bimetallic timer28 moves far enough to the left for the contact blade 24 to drop off thebutton 21 and shut the system down as in the case of ignition failure.

It may be noted that this system has an addisume a positioncorresponding to said second condition value whereby said timer isdeenergized.

3. ma fuel control device, in combination, an

I electrically operable "fuel supply controlling device, a circuittherefor, an electric timingdevice, connections by which said timingdevice, when energized, first closes said circuit and thereafter opensit again, means responsive to combustion conditions cooperating withsaid timing device to prevent the reopening of said circuit ifcombustion is established first, a thermal element, a first switchclosed by said thermal element at a first temperature, a second switchclosed by said thermal element at a second lower temperature, a circuitenergizing said timing device including both switches, a third switchclosed by said timing device when energized, a local heater. for saidthermal element, and circuit connections ener-- gizing said heater andmaintaining said timing device energized when said second switch is openincluding said first and third switches, said heater causing thetemperature of said thermal element to increase to said first valueafter a predetermined length of time.

GEORGE H. FISHER.

